Mk 139 (Obsolete)

Rocket uses in

Functioning

Arming time

Body diameter, inches

Over-all length, inches

4.25

General: This fuze was designed for antisubmarine warfare and was used in rockets projected both from airplanes and from small patrol craft. The fuze functions on impact with a hard object, but not on water impact.

Description: The fuze body encloses the functioning mechanism. A waterproof cap is affixed to the nose of the fuze and is held in position by two bands locked by a safety pin. Beneath the cap, a firing wheel is attached by a setscrew to the end of a firing pin. This wheel is formed like a cross to present less surface and prevent firing on water im-pact. The slide stop pin, riveted to the cap, engages a leg of the firing wheel. This pin also extends down into the nose plate, where it prevents a slide from moving outward. A flywheel and set-back collar, separated by a spring and secured by the flywheel screw, form a subassembly. This assembly is placed on the shaft of the firing pin. A pin set in the slide engages the set-back collar in order to prevent its rotation. A clock spring, se-cured on one end to the flywheel and on the other end to a pin on the closure disc, is assembled under tension so as to impart its force to the flywheel. The pin to which the clock spring is attached is anchored to the fuze body to prevent rotation of the closure disc. The firing pin is screwed into shear threads in the closure disc. A detonator shutter is affixed on a pin set in the lower suface of the closure disc. The firing pin extends into a cavity in the shutter. A tightly fitted spacer ring set in the body maintains a spring de-tent housed in the detonator shutter. This ring, interposed between the closure disc and lead-in disc, provides free movement of the shutter. The lead-in charge is contained in the lead-in disc, and the booster charge is a booster magazine which screws into the fuze body.

Operation: When the arming wire is withdrawn as the rocket is launched, the clamps are unlocked and forced off by the clamp spring. The waterproof cap flies off by the ac-tion of its compressed springs. Set-back causes the set-back collar to move back a-gainst its spring. This movement releases the slide pin in the nose cap, permitting the flywheel assembly to be rotated by the force of the clock spring. This rotation is trans-mitted to the firing pin, causing it to screw outward, thereby withdrawing the firing pin from engagement with the detonator shutter, which is forced by its spring into alignment with the firing pin. Once the shutter moves into the armed position, it is locked by a spring-loaded detent.

On impact of the firing wheel with a solid object, the firing pin is driven backward and shears the shear threads of the closure disc. It then pierces the detonator, setting off the explosive train.

Figure 152. Navy Nose Fuze (Rocket) Mk 139